Boron nitride nanocomposite membrane for organic pollutant and salt removal from water

Abstract

A desalination plant consists of pretreatment and the main salt rejecting units. A pretreatment unit consists of ultrafiltration membranes for filtering organic matter from water. While the salt rejecting unit, the heart of desalination process, consists of reverse osmosis (RO) membranes. In this study, a potential membrane material to be used for pretreatment and desalting units in the desalination plant. Ultrafiltration polysulfone-based mixed matrix membranes (MMMs) incorporated with two- dimensional boron nitride nanosheet (BNNS) were prepared via phase inversion method. The amount of BNNS incorporated was varied and the influence on membrane morphology, contact angle, surface charge, as well as water permeability and organic matter rejection were investigated. Results revealed that the addition of BN to the membrane matrix resulted in profound increase in water permeability (almost tripled to that of neat polysulfone membrane (PSf) , as increase from 50 L/m2.h.bar for neat PSf to 110 L/m2.h.bar for PSf/BN 1.0%) and humic acid rejection due to the increase in pore size and surface negative charge (94% of humic acid rejection for PSf/BN 1.0%). Beyond the morphological changes imparted by the inclusion of BNNS, the presence of BNNS within the membrane matrix also contributed to the enhancement in flux and humic acid rejection based on surface- slip and selective interlayer transport in the membrane. Despite the favourable augmentation of water transport and filtration performance, the MMMs suffered from fouling problem due to the entrapment of foulant within the enlarged pores and the membrane valleys. Inherent adsorptive character of BNNS could be a disadvantage when utilized as a membrane filler without proper modification. Next, raw BNNS and chemically activated BNNS (A-BN) were used as a nanofiller for the development of thin film nanocomposite (TFN) membrane. TFN membranes were synthesized via interfacial polymerization reaction between 1,3-Phenylendiamine (MPD) and trimesoyl chloride (TMC) monomer to form ultra-thin polyamide layer on PSf porous substrate. TFN membrane was responsible for desalination through RO process. Results showed that TFN membranes consisted of BNNS and A-BN as nanofillers have shown improvement in water permeability (33.9% higher than TFC membrane), with a minor loss in sodium chloride (NaCl) rejection (89.84% NaCl rejection for TFN-4). A comparative study with TFN membrane consisted of A-BN revealed a strong influence in improving water permeability with a minimal loss in salt rejection compared to TFN membrane with raw BNNS.